Microelectromechanical structures (MEMS) and other microengineered devices are presently being developed for a wide variety of applications in view of the size, cost and reliability advantages provided by these devices. Many different varieties of MEMS devices have been created, including microgears, micromotors, and other micromachined devices that are capable of motion or applying force. These MEMS devices can be employed in a variety of applications including hydraulic applications in which MEMS pumps or valves are utilized, optical applications which include MEMS light valves and shutters, and electrical applications which include MEMS relays.
MEMS devices have relied upon various techniques to provide the force necessary to cause the desired motion within these microstructures. For example, controlled thermal expansion of an actuator or other MEMS component has been used to actuate MEMS devices. See, for example, U.S. Pat. No. 5,909,078 and U.S. patent application Ser. Nos. 08/936,598 and 08/965,277, all assigned to MCNC, also the assignee of the present invention, which describe MEMS devices having thermally actuated microactuators, the contents of which are incorporated herein by reference.
An example of a thermally actuated microactuator for a MEMS device comprises one or more arched beams extending between a pair of spaced apart supports. Thermal actuation of the microactuator causes further arching of the arched beams which results in useable mechanical force and displacement. The arched beams are generally formed from nickel using a high aspect ratio lithography technique which produces arched beams with aspect ratios up to 5:1. Although formed with high aspect ratio lithography, the actual nickel arched beams have rather modest aspect ratios and may therefore have less out-of-plane stiffness and be less robust than desired in some instances. Further, the lithography technique used to form nickel arched beams may result in the arched beams being spaced fairly far apart, thereby increasing the power required to heat the arched beams by limiting the amount that adjacent arched beams heat one another. In addition, the resulting microactuator may have a larger footprint than desired as a result of the spacing of the arched beams. Thus, there exists a need for arched beams having higher aspect ratios in order to increase the out-of-plane stiffness and the robustness of microactuators for MEMS devices. In addition, there is a desire for microactuators having more closely spaced arched beams to enable more efficient heating and a reduced size.
Nickel microactuators are typically heated indirectly, such as via a polysilicon heater disposed adjacent and underneath the actuator, since direct heating of the nickel structure (such as by passing a current therethrough) is inefficient due to the low resistivity of nickel. However, indirect heating of the microactuator of a MEMS device results in inefficiencies since not all heat is transferred to the microactuator due to the necessary spacing between the microactuator and the heater which causes some of the heat generated by the heater to be lost to the surroundings.
Nickel does have a relatively large coefficient of thermal expansion that facilitates expansion of the arched beams. However, significant energy must still be supplied to generate the heat necessary to cause the desired arching of the nickel arched beams due to the density thereof. As such, although MEMS devices having microactuators with nickel arched beams provide a significant advance over prior actuation techniques, it would still be desirable to develop MEMS devices having microactuators that could be thermally actuated in a more efficient manner in order to limit the requisite input power requirements.
Thermally actuated MEMS valve structures having nickel arched beam actuators typically also have valve plates comprised of nickel. Since the limited aspect ratios attainable with nickel results in structures similarly limited in out-of-plane stiffness and robustness, MEMS valves having nickel valve plates are generally restricted to lower pressure fluid systems in order for the valve to operate with acceptable sealing. While out-of-plane stops for the valve plates may be helpful in increasing the pressure capabilities of a MEMS valve, stops are typically difficult to construct using conventional semiconductor processing techniques for MEMS valves having nickel valve plates. Thus, there exists a further need for more robust MEMS valves with valve plates having increased out-of-plane stiffness and thus for application in higher pressure fluid systems. In addition, it would be desirable for the valve construction to facilitate the formation of out-of-plane stops using conventional semiconductor processing techniques, wherein the stops would contribute to the stability and sealing ability of the valve plate.